Epitaxial growth is typically used to form a high quality layer on a substrate. In epitaxy crystal growth, a high growth rate and a smooth surface are desirable. A high capacity of the reactor in which growth takes place is usually also desirable.
At atmospheric pressure, it is difficult to reliably form a quality layer on a large substrate. The surface morphology of a layer grown at atmospheric pressure is usually poor on account of a high degree of homogeneous nucleation. Homogeneous nucleation is the effect you obtain when one or several precursors react and grow in the gas phase to form clusters or micro-crystals of material. Silane can, for instance, decompose thermally and small micro-crystals of silicon will grow if the concentration of silane is high. A reduction in pressure and/or increase of hydrogen carrier flow improves the situation significantly, however, etching of the SiC surfaces due to the hydrogen will increase markedly.
For silicon carbide, the standard precursors are silane and a hydrocarbon together with hydrogen as a carrier gas. Unfortunately, as the reactors have become larger, the need to put in more silane has resulted in a higher degree of homogeneous nucleation, which manifests itself as poor morphology, and, in severe cases, as particles on the surface, often mistaken as particle downfall from the walls of the reactor/susceptor. Very often homogeneous nucleation can be observed with the naked eye (screened by an equal density filter for the intensity) as whisps of smoke in the reactor.
If the concentration of silane is too high (a high supersaturation) it literally “rains” silicon, forming boulders on the substrate surface, or, if the concentration is somewhat lower, the homogeneous nucleation creates a wavy effect on the surface.
These problems may be reduced by adding more hydrogen, going to lower pressure, and/or modifying the inlet of the gases. The addition of more hydrogen, especially in combination with low pressure, unfortunately increases the etch rate of the SiC which reduces the net growth rate. It will also stress the materials inside the reactor, particularly the graphite materials and may create holes in the coating thereby releasing lots of impurities. The high gas flows at low pressure will also increase the size of the pump, pump lines, and cooling of the reactor parts which cost significant amounts of money. The high gas flows also markedly cool the front of the reactor, which often results in a reduced uniformity of the layer. Addition of argon replacing some of the hydrogen can improve the uniformity, however, argon addition tends to amplify homogeneous nucleation problems, making its use impractical for large reactors.
An interesting growth technique referred to as the “chimney technique” invented by Alex Ellison uses a very low flow of hydrogen and a fairly low pressure of 200 mbar. The system is vertical with the inlet at the bottom, but due to the reduced pressure, buoyancy is not a dominant force and therefore the inlet can be from the top equally well. The principle of the technique is that the silane cracks to form silicon clusters in the inlet region. These clusters of silicon are transported via the carrier flow towards the substrate. As the temperature increases (the substrate temperature is between 1700° C.-1900° C.) the silicon clusters dissociate and become available for growth. It is important that the gas is heated enough so that the silicon clusters dissociate. In a horizontal reactor, this method does not work unless the samples are placed upside down. If they are not placed upside down, large boulders created by silicon droplets falling down will be manifested on the surface. Growth rates with this technique were in the order of 10-50 microns/h using minor amounts of silane gas. The doping was also very low, but due to the high temperature, the carrier lifetimes were poor. The morphology was very good on some places but it was unfortunately not uniform over the whole wafer surface.
Accordingly, a need exists to reduce the degree of homogeneous nucleation preferably with lower gas flows and higher pressures while maintaining adequate growth rate and crystal uniformity.